Split off a separate --harmony_sloppy_let flag

test/mjsunit/harmony/block-scoping-sloppy.js

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-// Flags: --allow-natives-syntax --harmony-sloppy --no-legacy-const
+// Flags: --allow-natives-syntax --harmony-sloppy --no-legacy-const --harmony-sloppy-let
 // Test functionality of block scopes.
 
 // Hoisting of var declarations.
 function f1() {
   {
     var x = 1;
     var y;
   }
   assertEquals(1, x)
   assertEquals(undefined, y)
@@ skipping 221 matching lines @@
     const_accessors[i] = function() { return z; }
   }
   for (var j = 0; j < 10; j++) {
     y = j + 10;
     assertEquals(j, let_accessors[j]());
     assertEquals(y, var_accessors[j]());
     assertEquals(j, const_accessors[j]());
   }
 }
 f8();